1. Field of the Invention
The present invention relates to semiconductor devices, and, in particular, to integrated image/light sensors, such as MOS image sensors, having a light-shielding layer.
2. Description of the Related Art
Image sensors typically employ an array of light-sensing elements (e.g., photodiodes) to generate electrical signals representing light incident on the device. In addition to the array of light-sensing elements, an image sensor typically includes associated circuitry for selectively reading out the electrical signal generated by the individual elements. The light-sensing elements operate by the well-known photoelectric effect by which the incidence of photons of light on each element generates electrons that constitute the electrical signal from that element. Image sensors may be implemented using color filter arrays (e.g., comprising distributions of different color elements, each of which is responsive to one of red, green, and blue light) to generate color output.
As CMOS technology scales down to deep submicron regimes, the aspect ratio becomes smaller. Here, the aspect ratio is the ratio of the width of an opening in a light-shielding layer and the vertical distance from the light-shielding layer to the corresponding light-sensing element. As the aspect ratio decreases, more and more xe2x80x9coff axisxe2x80x9d light (i.e., light incident at a non-normal angle) cannot reach the light-sensing element, resulting in the so-called pixel vignetting phenomenon. This often leads to a significant reduction in sensor sensitivity. It can also result in significant non-uniformity in pixel response over the sensor array.
FIG. 1A shows a cross-sectional view of a portion of an image sensor 100 of the prior art. Image sensor 100 has a transparent dielectric layer 102 formed over a substrate 104. Disposed within dielectric layer 102 is an opaque light-shielding layer 106 having a pattern of transparent openings 108 that selectively allows incident light to pass through dielectric layer 102 towards an array of light-sensing elements 110 formed on substrate 104 at the interface between dielectric layer 102 and substrate 104. Each light-sensing element 110 corresponds with one of the transparent openings 108 in light-shielding layer 106.
In an ideal situation, light always impinges upon image sensor 100 at a normal angle (i.e., perpendicular to both light-shielding layer 106 and the array of light-sensing elements 110). In that case, all of the light that passes through each transparent opening 108 in light-shielding layer 106 reaches the corresponding light-sensing element 110 below. In reality, however, depending on the situation, at least some if not all of the light is incident at non-normal angles and, in those cases, at least some of the light passing through a transparent opening 108 does not reach the corresponding light-sensing element 110.
FIG. 1A shows an exemplary situation of a light source (not shown) positioned at a relatively close distance directly over image sensor 100. In this case, essentially all of the light that passes through transparent opening 108c reaches corresponding light-sensing element 110c. However, due to the relatively small aspect ratio of image sensor 100, some of the light passing through transparent openings 108b and 108d will not reach corresponding light-sensing elements 110b and 110d, respectively, and even less of the light passing through transparent openings 108a and 108e will reach corresponding light-sensing elements 110a and 110e. The light falling outside of the light-sensing elements 110 will impinge on other regions 112 of the substrate 104 between those light-sensing elements 110, which regions 112 might not be light sensitive and will, in any case, not contribute appropriately to the electrical signal generated by light-sensing elements 110.
FIG. 1B illustrates the pixel vignetting that results from the geometry shown in FIG. 1A for off-axis elements, such as elements 110a and 110e. In particular, FIG. 1B shows a top view of the active area of a light-sensing element 110 and the actual light pattern 114 that impinges on substrate 104 after passing through the corresponding transparent opening 108. As shown in FIG. 1B, light pattern 114 is offset from element 110. This offset reduces the sensitivity of this element in the image sensor to such light.
The present invention addresses the problem of pixel vignetting by providing an image sensor having a reflective optical path between the light-shielding layer disposed within the transparent dielectric layer and each of one or more light-sensing elements formed on the substrate below. Each reflective optical path is preferably implemented using a reflective liner formed in a via between the light-shielding layer and a corresponding light-sensing element. The reflective paths help to direct non-normal incident light towards the appropriate light-sensing elements, thereby enhancing sensor sensitivity as well as uniformity of response across the sensor array.
In one embodiment, the present invention is an integrated sensor comprising (a) one or more light-sensing elements formed on a substrate; and (b) a light-reflecting structure for each light-sensing element, configured within intervening structure above the substrate to reflect incident light towards the light-sensing element.